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ZnS/ZnO Heteronanostructure as Photoanode to Enhance the Conversion Efficiency of Dye-Sensitized Solar Cells Xue-Lian Yu,†,‡ Jun-Guo Song,† Ying-Song Fu,†,‡ Yang Xie,† Xin Song,† Jing Sun,† and Xi-Wen Du*,† School of Materials Science and Engineering, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, and Motorola (China) Electronics Ltd., No. 10, Fourth AVenue, TEDA, Tianjin 300457, People’s Republic of China ReceiVed: October 29, 2009; ReVised Manuscript ReceiVed: December 11, 2009
ZnS/ZnO heteronanostructures were prepared to serve as the photoanode of the dye-sensitized solar cells. Two nanostructures, namely, ZnS/ZnO coaxial nanowires and ZnS/ZnO hierarchical nanowires (ZnS nanoparticles on ZnO nanowires), were successfully synthesized by chemical bath deposition and chemical etching processes, respectively. For both of the nanostructures, the ZnS coating can enhance photocurrent and conversion efficiency compared with the bare ZnO nanowires. We propose that ZnS layers in the two nanostructures take effect in different ways in that the ZnS compact layer in the coaxial structure retards the back transfer of electrons to the dye and electrolyte, while the coarse surface of ZnS nanoparticles in the hierarchical nanowires significantly enhances the adsorption of dye molecules. Hence, the ideal photoanode structure for high power-conversion efficiency should have both the compact shell layer and the high surface roughness. 1. Introduction ZnO is being intensely investigated as a photoanode material in dye-sensitized solar cells (DSSCs) because of its similarities with the traditional one, TiO2; that is, it is a wide band-gap semiconductor oxide (3.37 eV) with a conduction band edge located at approximately the same level as that of TiO2.1 Apart from this, ZnO is more attractive for its application in DSSCs because the electron mobility of ZnO has been proven to be higher than that of TiO2, which means lower charge recombination.2 Another important advantage of ZnO over TiO2 is that it can be fabricated with many synthesis techniques to obtain a great variety of morphologies, especially vertically aligned nanostructures.3-7 Some of methods are easily scalable and suitable for many substrates,8 which makes ZnO very attractive for fabricating DSSCs by adopting low-cost solution techniques completely. However, despite its great potential, the convention efficiency of ZnO-nanowire-based DSSCs remains only around 0.5-1.5% at present.9-11 One reason for the low conversion efficiency of ZnO-based DSSCs is the recombination of the electrons injected into the ZnO with either the dye or the redox electrolyte, thereby reducing the cell efficiency.12 In the TiO2 nanoporous system, a common technique to reduce the recombination rate is to create core-shell heterostructures. The energy barrier formed between the core and shell materials can hinder the recombination process.13-15 Another reason may be explained by the chemical instability of ZnO in acid dye. The -COOH group of the dye dissolves ZnO to from a [dye-Zn2+] complex, which blocks the electron injection from the dye to ZnO.16 In the present work, we first fabricate the ZnO nanowire arrays by a hydrothermal route and then introduce the ZnS shell onto the ZnO nanowire to overcome the above two limitations. To illustrate the effect of the ZnS layer, two nanostructures, * To whom correspondence should be addressed. E-mail:
[email protected]. † Tianjin University. ‡ Motorola (China) Electronics Ltd.
ZnS/ZnO coaxial nanowires and ZnS/ZnO hierarchical nanowires, were designed and produced by chemical bath deposition and chemical etching processes, respectively. By varying the thickness and roughness of the ZnS layer, we made a systematic study on the factors that influence the photovoltaic properties of ZnO-based DSSCs. The morphology and structure of the ZnS layer show remarkable influence on the final performance of the DSSCs, which indicates that the conversion efficiency can be further improved by careful design of the photoanode materials. 2. Experimental Section The preparation process of ZnO nanowire arrays includes substrate pretreatment and hydrothermal deposition, similar to that described in the literature.17 To prepare ZnO nanowires, ZnO nanoparticles were deposited as a seed layer on FTO by a dip-coating method, where the precursor solution for dip-coating was prepared by using zinc acetate dehydrate [Zn(CH3COO)2, 0.75 M] as the starting material, ethanolamine (MEA, NH2CH2CH2OH, 0.75 M) as the stabilizer, and 2-methoxyethanol (CH3OCH2CH2OH) as the solvent. ZnO nanowire arrays were then grown in the aqueous solution of 0.05 M zinc nitrate hexahydrate and 0.05 M hexamethylenetetramine and heated at 90 °C for 2 h. Finally, the as-grown products were annealed in air at 450 °C for 2 h. Chemical bath deposition (CBD) was performed to deposit ZnS layers onto the ZnO nanowires and obtain the ZnS/ZnO coaxial structure. Typically, the samples were successively immersed in two different solutions, 5 mM Zn (NO3)2 and another containing 5 mM Na2S, at 60 °C for a period of time. After the CBD process, the samples were rinsed with distilled water, dried, and annealed in air at 300 °C for 10 min. A chemical etching process (CEP) was adopted to prepare the ZnS/ZnO hierarchical nanostructures. The as-prepared ZnO nanowires were immersed in thiacetamide (CH3CSNH2) aqueous solution (5 mM) and heated on a hot plate at 90 °C for a period
10.1021/jp910355m 2010 American Chemical Society Published on Web 01/12/2010
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Figure 1. (a) Top-view and (b) cross-sectional SEM images of the ZnO nanowire arrays. (c) SEM and (d) HRTEM images of the ZnS/ZnO coaxial nanostructure; the inset of (d) is the TEM image of the ZnS/ZnO coaxial nanostructure. (e) SEM and (f) HRTEM images of the ZnS/ZnO hierarchical nanostructure; the inset of (f) is the TEM image of the ZnS/ZnO hierarchical nanostructure.
of time. The resultant product was then rinsed with distilled water, dried, and annealed for further characterization. Before solar cell testing, the FTO substrates with ZnS/ZnO nanostructures were heated to 70 °C and sensitized in standard ruthenium-based N3 red dye with a concentration of 5 × 10-4 M in ethanol for 30 min. The samples were then rinsed with ethanol to remove excess dye on the surface and air-dried at room temperature. The DSSCs were constructed by using the FTO substrate with ZnS/ZnO nanostructures as photoanodes, Pt-coated FTO glass as a counter electrode, and an iodide-based solution as the liquid electrolyte. Current-voltage (I-V) characteristics were recorded using a Keithley 2611 digital multimeter. The light source was a 150 W xenon lamp (Sciencetech, SS150) calibrated to 100 mW/cm2 with a radiometer. The morphology of the samples was observed using a Hitachi S-4800 scanning electron microscope (SEM) and an FEI Tecnai G2 F20 transmission electron microscope (TEM) with a field emission gun operating at 200 kV. TEM samples were prepared by dropping dilute products onto carbon-coated copper grids. The X-ray diffractometry (XRD) for the crystal structure of the products was carried out in a Rigaku D/max2500v/pc diffrac-
tometer. UV-vis absorption spectra were recorded on a Hitachi 3010 spectrophotometer. 3. Results and Discussion A typical SEM image of the ZnO nanowire arrays is shown in Figure 1. The top view (Figure 1a) shows that ZnO nanowires distribute uniformly and compactly on the FTO substrate. The side view (Figure 1b) clearly shows that ZnO nanowires are aligned with the diameter of about 50 nm and thickness of about 2.5 µm. The morphology changes of the ZnO nanowire arrays are observed by SEM and HRTEM after the CBD and CEP, respectively. The image of the ZnS/ZnO coaxial nanostructure after the CBD process is shown in Figure 1c,d. ZnO nanowires are coated with a thin layer of ZnS to form a coaxial nanostructure (Figure 1c), and the contrast between the edge and center in the TEM image (Figure 1d) is indicative of the coaxial core-shell structure, with the shell thickness about 5 nm after 0.5 h deposition. The HRTEM image further demonstrates the highly crystalline (001) direction-oriented ZnO core and the composition of ZnS nanocrystals in the shell. Different
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SCHEME 1: Energy Level Diagram for the ZnS/ZnO Heterostructures with N3 Dye
from the CBD process, the surface morphology of ZnO nanowires becomes coarse after the CEP, as shown in Figure 1e. The well-ordered nanowires still exist, suggesting that the etching process does not damage the arrays. From the TEM image in Figure 1f, we can see that ZnS nanoparticles with the size of several nanometers disperse sparsely on the ZnO nanowires. These nanoparticles were formed by the reaction of S2- anions with the dissolved Zn2+ ions from the surface of ZnO nanowires. As the etching time increases, the roughness of the surface increases, too. The observed 0.31 nm spacing of the nanoparticle corresponds to the (111) plane of the ZnS cubic phase. The crystal structure of the ZnS/ZnO coaxial nanowires was confirmed by X-ray diffraction and energy-dispersive spectroscopy (EDS). As shown in Figure 2a, after the CBD process, heterostructured composites were formed. In addition to hexagonal structure ZnO (JCPDS 79-2205), new peaks appear, which can be indexed to the cubic phase ZnS (JCPDS 79-0043). Furthermore, the spatial distributions of the atomic composition across the ZnS/ZnO coaxial nanowire were obtained by a nanoprobe EDS line-scan analysis (marked by a line in Figure 2b), showing that the ZnO nanowire was homogeneously coated
Figure 2. XRD (a) and STEM image (b) of ZnS/ZnO coaxial nanostructures. (c) The profiles of line-scan analysis across the ZnS/ ZnO coaxial nanowire, as indicated by the line in (b).
by a thin ZnS shell, as shown in Figure 2c. All of these results definitely demonstrated the formation of the ZnS/ZnO coaxial nanostructure. The photoelectrochemical behaviors of DSSC structures were measured at an illumination intensity of air mass 1.5. The photocurrent density-voltage characteristics of the coaxial and hierarchical structures are shown in Figure 3a,b, respectively, and the related physical values, such as JSC (short-circuit current), VOC (open-circuit voltage), FF (fill factor), and η (lightto-electricity conversion efficiency), are summarized in Table 1. The η value can be evaluated from the equation17
η ) (FF × JSC × VOC)/Pin
(1)
where Pin is the incident light power density. For the bare ZnO nanowire arrays, the η value is about 0.26%, whereas for the ZnS/ZnO hierarchical nanostructure, the efficiency can be improved to 0.42% after ZnS etching at an optimal time (1 h). For the ZnS/ZnO coaxial structure, it exhibits the highest efficiency of 0.76% as the reaction time keeps 0.5 h and the shell thickness is of about 5 nm from the TEM observation; further increasing the CBD time results in poorer conversion efficiency than that of the controls. All of these results indicate that, under an optimal reaction time, ZnS/ZnO heteronanostructures can enhance photocurrent and efficiency compared with the bare ZnO nanowires. To understand the efficiency enhancement of DSSCs with ZnS/ZnO hierarchical nanostructures, we measured the optical absorbance of the samples after being dye-sensitized for 30 min. It can be seen from Figure 4 that all of these nanostructure arrays show a wide absorbance band in the visible region (centered about 510 nm), and the hierarchical nanowires show higher absorption than that of bare nanowires and coaxial nanowires at the same sensitized time. We believe that this increased photoabsorption is the main reason for the improved conversion efficiency. For hierarchical nanowires, as etching time goes by, the surface of the ZnO nanowire becomes coarser due to more ZnS nanoparticles formed. In principle, the coarse surface can increase series resistance and decrease current density. On the other hand, the coarse surface can enhance the dye adsorption as well as diminish the reflection of incident light so as to increase the volume of the optically active component,18 which can lead to the increase in current density. The fact is that the highest short-circuit current appears to be 2.2 mA/cm2 when the etching time is as long as 1 h (shown in Table 1); this result indicates that photoabsorption plays a more important role on the final conversion efficiency than series resistance. However, the final conversion efficiency of coaxial nanowires is much higher than that of hierarchical nanowires. We propose that the recombination of electrons in ZnO nanowires with dye or electrolyte is the root cause for the limited enhancement of
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Figure 3. I-V characteristics of the solar cells made from (a) ZnS/ ZnO coaxial nanostructures. (b) ZnS/ZnO hierarchical nanostructures at different reaction times.
TABLE 1: Photocurrent-Voltage Characteristics of ZnO-based DSSCs samples
VOC (mV)
JSC (mA/cm2)
FF
η
ZnO etching 0.5 h etching 1 h coaxial 0.5 h coaxial 1 h
625 540 504 677 640
1.14 1.5 2.2 1.87 0.92
0.36 0.41 0.38 0.6 0.42
0.26 0.33 0.42 0.76 0.25
conversion efficiency for hierarchical nanowires, whereas such a disadvantage can be effeciently depressed in coaxial nanowires. The energy band structure of ZnO, ZnS, and N3 dye can be shematically illustrated in Scheme 1, where the conduction band edge of ZnS is higher than that of ZnO and the lowest unocuppied molecular obital (LUMO) of dye; thus, the compact ZnS shell is very efficient on retarding the back transfer of electrons and minimizing electron-hole recombination, which significantly decreases the dark current and is beneficial for the final conversion efficiency. Besides, the ZnS shell prevents the direct contact of dye with ZnO nanowires, thus further decreasing the formation rate of a [dye-Zn2+] complex. According to the above results, although the hierarchical nanostructure has a higher surface area and adsorbs much more dye molecules, ZnS nanoparticles are separated and cannot prevent the back transfer of electrons; therefore, the enhancement of final conversion efficiency is very limited (from 0.26% to 0.42%). On the contrary, the photoabsorption of the coaxial nanostructure does not improve obviously as compared with the bare nanowires, but the compact ZnS shell can retard the
Figure 4. (a) Light absorbance spectra of the samples after being dyesensitized for 30 min. (a) ZnS/ZnO hierarchical nanostructures. (b) ZnS/ ZnO coaxial nanostructures.
back transfer of electrons efficiently; hence, the final conversion efficiency of the coaxial nanostructure is much higher than that of bare nanowires (from 0.26% to 0.76%). We then conclude that the prohibition of electron back transfer by the coaxial nanostructure is more crucial than the enlargement of dye adsorption by the hierarchical structure, but both of the two efforts are helpful for the higher conversion efficiency of DSSCs. 4. Conclusion ZnS/ZnO heteronanostructures were prepared to serve as the photoanode of DSSCs. Two nanostructures, ZnS/ZnO coaxial nanowires and ZnS/ZnO hierarchical nanowires, were obtained by chemical bath deposition and chemical etching processes, respectively. A systematic investigation was made on the influence of the thickness and roughness of the ZnS layer on the photovoltaic properties of ZnO-based DSSCs. The coaxial nanostructures show high capability on the prohibition of electron back transfer and achieve higher photoelectron conversion efficiency, and the rough surface of hierarchical nanostructures is also beneficial for the improvement of the performance of DSSCs. Acknowledgment. This work was financially supported by the National High-tech R&D Program of China (Nos. 2007AA021808 and 2009AA03Z301), the Natural Science Foundation of China (No. 10732020 and No. 50672065), and the National Science Foundation for Postdoctoral Scientists of China (20090450089). References and Notes (1) Quintana, M.; Edvinsson, T.; Hagfeld, A.; Boschloo, G. J. Phys. Chem. C 2007, 111, 1035.
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JP910355M
